Let's say put a file on the desktop, and when it opens, it opens in the terminal. If I want to do this, what extension can I use in "Save As" of the Text Editor? (e.g. open.shell)
Ubuntu 12.04.2 64-bit

The terminal is not for open files with it! You can edit/open/view files from inside of the terminal using commands like nano file_name/xdg-open file_name/cat file_name.

Also (maybe about this thing you are interested) you can run directly in terminal some executable text files (like bash script files). To do this, open Nautilus, go to Files → Preferences, select Behaviour tab and select the option you prefer for executable text files: